181982937-Clark-s-Pocket-Handbook-For-Radiographers-pdf (1)

‘The 10-Point Plan’
5 Correct image exposure: the evaluation of the appropriateness of
the exposure factors used for a radiograph will depend on the
equipment used to obtain the image.
Conventional Film-based Imaging
■ Image density: the degree of film blackening should allow relevant
anatomy to be sufficiently demonstrated, thus allowing diagnosis.
■ Image contrast: the range of useful densities produced on the
radiographic image should correspond to the structures within
the area of interest. Each anatomical area should be of sufficient
density to allow relevant anatomical structures to be clearly
visualized.
Digital Image Capture Systems
Given the wide exposure latitude of digital systems the primary task
when evaluating the image is to assess for over- and underexposure.
The imaging equipment will usually give a numerical indication of
the exposure used, the detector dose indicator (DDI). The reading is
compared with a range of exposure limits provided by the manufacturer
to see if it is above or below recommended values.
Underexposure: images that are underexposed will show unacceptable
levels of ‘noise’ or ‘mottle’ even though the computer screen brightness
(image density) will be acceptable (see image, page 27).
Overexposure: image quality will actually improve as exposure
increases due to lower levels of noise. Once a certain point is reached,
further increases in exposure will result in reduced contrast. Eventually
a point is reached when the image contrast becomes unacceptable (see
image, page 27).
NB: There is considerable scope for exposing patients to unnecessarily
high doses of radiation using digital imaging technologies. When
evaluating images it is important to always use the lowest dose that
gives an acceptable level of image noise and to compare doses used with
national dose reference levels.
6 Optimum resolution: is the image sharp? Look at bone cortices and
trabeculae to ensure movement or other factors have not caused an
unacceptable degree of unsharpness.
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